1. Field of the Invention
The present invention relates to neutron absorbing material and, more particularly, to a porcelain enamel material having neutron absorbing properties used for coating processing and storage equipment for use in the nuclear industry.
2. Description of the the Prior Art
The design of chemical process equipment, such as piping, retention or reaction vessels, material handling and storage equipment, for nuclear industrial operation requires a consideration of nuclear criticality whenever there is a possibility of the presence of fissile material in the process stream. A criticality is an uncontrolled nuclear reaction resulting in an intense release of radiation and heat. In the absence of shielding, a criticality presents a high potential for death of persons nearby (within 50 feet). The political or public impact of such an event is even greater. Public reaction to such an event has resulted in extensive facility shutdowns and lawsuits. Even near criticality incidents have resulted in similar public reaction. It is therefore imperative that the nuclear industry do whatever is necessary to prevent unplanned nuclear criticality.
Basically, the known physical and nuclear parameters are used to assure that accumulations of fissile material are maintained subcritical by geometry and mass control. In addition to these physical parameters in the structural design of equipment, the use of neutron absorbing materials (also referred to as "neutron poisons") can increase the level of criticality control associated with a piece of equipment.
One approach to providing a neutron absorbing protection is to fabricate the process equipment of a material that includes one or more known neutron poisons such as boron, cadmium, hafnium, and gadolinium. For example, stainless steel alloy that contains boron as a constituent has been commonly used for the fabrication of reaction vessels, piping and storage racks in the nuclear industry. The use of borated stainless steel, however, is very expensive because its fabrication requires special alloy melts, castings and in some cases extensive machining.
There is also uncertainty about the corrosion properties of the specially tailored alloys that may require extensive corrosion testing. For example, borated stainless steel equipment might have to be replaced sooner on the basis of a small acceptable wall loss for criticality control rather than because of a lack of physical integrity of the equipment.
Another approach, disclosed in U.S. Pat. No. 4,298,579 is a centrally arranged neutron absorber rod in a tank containing a plutonium solution. The disclosure of this patent also suggests providing a tank wall with neutron absorbing material to be applied, in one instance, as a separate layer in the form of a suitable enamel. The disclosure, however, does not provide any details concerning the proposed enamel nor is there any suggestion concerning the form, composition or properties of the enamel layer.
Glasses containing boron, cadmium and other neutron absorbers are known in the nuclear industry. See, for example Sun et al., "Neutron Absorbing and Transmitting Glasses", The Glass Industry, 1950, 31 (10) pp. 507-515 and Melnick, et al., "Neutron-Absorbing Glass: CdO-SiO.sub.2 -B.sub.2 O.sub.3 System", Journal of the American Ceramic Society, 1951, 34(3) pp. 82-86. Such glasses while useful in a small scale laboratory setting may not meet the physical integrity requirements for large scale processing and storage equipment and are not suitable for application as porcelain enamel coatings.